1. Field of the Invention
The invention relates to a method and apparatus for manufacturing metal alloys, more particularly to a method and apparatus for manufacturing a light metal alloy by the process of injection molding the metal alloy when it is in a thixotropic (semi-solid) state.
2. Description of the Related Art
One conventional method used to produce molds of metal alloys is the die cast method. The die cast method is disclosed in U.S. Pat. Nos. 3,902,544 and 3,936,298, both of which are incorporated by reference herein. The die cast method uses liquid metal alloys during casting and as a consequence, metal alloys produced from this method have low densities. Metal alloys having low densities are not desirable because of their lower mechanical strength, higher porosity, and larger micro shrinkage. It is thus difficult to accurately dimension molded metal alloys, and once dimensioned, to maintain their shapes. Moreover, metal alloys produced from die casting have difficulty in reducing the resilient stresses developed therein.
The thixotropic method improves upon the die casting method by injection molding a metal alloy from its thixotropic (semi-solid) state rather than die casting it from its liquid state. The result is a metal alloy which has a higher density than one produced from the die casting method.
A method and apparatus for manufacturing a metal alloy from its thixotropic state is disclosed in U.S. Pat. No. 5,040,589, which is incorporated by reference herein. A method of converting a metal alloy into a thixotropic state by controlled heating is disclosed in U.S. Pat. Nos. 4,694,881 and 4,694,882, both of which are incorporated by reference herein.
The system disclosed in U.S. Pat. No. 5,040,589 is an in-line system, in which the conversion of the metal alloy into a thixotropic state and the pressurizing of the same for the purposes of injection molding is carried out within a single cylindrical housing. With such a system, it is difficult to control the molding conditions, i.e., temperature, pressure, time, etc., and as a result, metal alloys of inconsistent characteristics are produced.
Moreover, the system of U.S. Pat. No. 5,040,589 requires that the metal alloy supplied to the feeder be in pellet form. As a consequence, if a mold of undesired characteristics are produced by its system, recycling of the defective molds is not possible unless the defective molds are recast in pellet form.
An improved system for manufacturing light alloy metals, which is capable of accurately producing molded metal alloys of specified dimensions within a narrow density tolerance, is desired. Further, a production process for light alloy metals which can consistently produce molded metal alloys of desired characteristics, and which can easily accommodate recycling of defective molds would represent a substantial advance in this art.
An object of the invention is to provide a method and apparatus for producing metal alloys through injection molding.
Another object of the invention is to provide an improved injection molding system for metal alloys which is capable of producing molded metal alloys of accurate dimensions within a narrow density tolerance.
Still another object of the invention is to provide an injection molding system for light alloy metals which is capable of producing light alloy metals of desired characteristics in a consistent manner.
Still another object of the invention is to provide an injection molding system for light alloy metals which accommodates recycling of defective molds easily.
These and other objects are accomplished by an improved injection molding system for metal alloys in which the steps of melting the metal alloy, converting the metal alloy into a thixotropic state, and injecting the metal alloy in the thixotropic state into a mold are carried out at physically separate locations.
The improved system comprises a feeder in which the metal alloy is melted and a barrel in which the liquid metal alloy is converted into a thixotropic state. An accumulation chamber draws in the metal alloy in the thixotropic state through a valve disposed in an opening between the barrel and the accumulation chamber. The valve selectively opens and closes the opening in response to a pressure differential between the accumulation chamber and the barrel.
After the metal alloy in the thixotropic state is drawn in, it is injected through an exit port provided on the accumulation chamber. The exit port has a variable heating device disposed around it. This heating device cycles the temperature near the exit port between an upper limit and a lower limit. The temperature is cycled to an upper limit when the metal alloy in the thixotropic state is injected and to a lower limit when the metal alloy in the thixotropic state is drawn into the accumulation chamber from the barrel.
A piston-cylinder assembly supplies the accumulation chamber with the pressure necessary to inject the metal alloy in the thixotropic state and with the suction necessary to draw in the metal alloy in the thixotropic state from the barrel.
Additional objects and advantages of the invention will be set forth in the description which follows. The objects and advantages of the invention may be realized and obtained by means of instrumentalities and combinations particularly pointed out in the appended claims.